What's the difference? help plz!

おんばんわ みなーさん。
I'm new to these forums and i want to learn japanese more.. i know some but i rate myself under "Good"..i have a question:
What's the difference between the kanji letters that is connected to each other in one hiragana letter..
e.g: あ is A in hiragana. 亜 is A in kanji. ア is A in katakana. 阿 is A in kanji too.
what i mean is when do we use 阿 and when do we use 亜?
i mean how can japanese people remember all these letters? i want to learn
ひらがな mainly .. so please help me!

Your user name doesn't exactly fill me with a philanthropic urge to help, but...

There are only 46 syllables in Japanese (Learn A Chart of 46 Different Hiragana Symbols)

hiragana and katakana are syllabaries; where each character is a syllable. Kanji are pictographic characters that first and foremost have meaning attached to them, and can have multiple readings.

So, the obvious time to use a kanji that is read あ is not when you want the sound "あ", but when you want the meaning associated with the character 亜 or 阿

Just worry about learning about kana for now, and tackle kanji once you have the syllabaries down.